1. Field of the Invention
The present invention relates to an apparatus for measuring a moisture content within a gas by using a laser beam. For example, the present invention relates to an absorption spectrometric apparatus for semiconductor production process that is used for monitoring the moisture content within a process gas at all times for suppressing the moisture concentration within the process gas to be less than or equal to a predetermined value in a semiconductor production line.
2. Description of the Related Art
In a semiconductor production process, various fine processings and treatments are carried out on a semiconductor substrate (silicon wafer) surface. During that time, various process gases are used, such as an etching gas, a reaction gas for epitaxial growth, and a reaction gas for CVD (chemical vapor deposition). It is known in the art that, when moisture is contained in these process gases, the process gases and moisture, or the substrate surface and moisture react with each other to produce an unnecessary byproduct, whereby the yield of the produced semiconductor decreases considerably.
For this reason, in order to ensure stability of the processing in a semiconductor production process, one considers attaching a process monitor to the semiconductor production apparatus. As one of these, a measure of attaching a monitor to a discharging outlet of the process gas of the semiconductor production apparatus and grasping the state of the semiconductor production process by monitoring the components of the discharged gas, thereby to improve the yield both within a lot and between the lots and to achieve stabilization of the process is proposed.
Typically, as the process gas, a highly reactive gas is often used. For this reason, various substances such as reactive byproducts including a highly corrosive gas, unreacted substances, and moisture are mingled within the discharged gas. In the case of monitoring the discharged gas, measures both in hardware and in software are needed in the measuring system thereof.
As a monitor of the discharged gas, particularly a moisture monitor is important. As a method of measuring the moisture concentration contained in the process gas, a quartz oscillation type method that measures the frequency change of a quartz oscillator, and an electrostatic capacitance type method that measures the electrostatic capacitance change by allowing the moisture within the gas to be adsorbed are known. Also, a laser aquameter that measures the moisture concentration by infrared spectrophotometry using a laser of variable wavelength type (See Japanese Patent Application Laid-open Nos. 05-99845 and 11-183366) is proposed.
The above-described laser aquameter is such that the moisture concentration is determined by the intensity of a laser beam at the moisture absorption wavelength by introducing a sample gas into a sample cell, allowing a laser beam having a predetermined wavelength to be incident into the sample cell, and analyzing a transmitted laser beam. Since the measurement can be made without letting the sensor part be in contact with the measurement object gas, the laser aquameter can be applied to a corrosive gas unlike those of the quartz oscillation type or the electrostatic capacitance type, and is also characterized in that the response speed is high.
Also, as a discharged gas analyzing cell in the above-described laser aquameter, in order to raise the analyzing sensitivity, those of a White type, a Herriott type in which reflection is made for plural times between two opposing mirrors, or the like are used (See D. R. Herriott, H. Kogelnik, and R. Kompfer, Appl. Opt. 3, 523 (1964)).